1. Field of the Invention
The present invention generally relates to measurements of fluid flows in downhole applications, more particularly to the use of measurements to determine gas ratios in the fluid flows, and other related applications.
2. Background Art
Natural gas hydrates, such as methane hydrates, may be formed from natural gas (e.g., methane) and water by the decomposition of microorganisms at low temperature and high pressure. Natural gas hydrates are abundant around the world, including large known deposits below permafrost and in deep sea beds. Energy-supply research estimates indicate that these natural gas hydrates contain more energy than all other fossil fuel deposits combined. However, technology needed to recover large amounts of natural gas from hydrate has proven to be economically challenging.
Research programs are underway to reform natural gas hydrate to produce large volumes of useable hydrocarbon fuels, particularly methane (CH4), by injection of liquid CO2 into the natural gas (CH4) hydrate to convert the natural gas hydrate into CO2 hydrate and CH4 gas. Thus, natural gas can be released in a form that can be readily recovered using conventional means. For example, Komai et al. (Preprints, Div. of Fuel Chemistry, ACS National Meeting 1997, San Francisco, 568-572) discuss the use of liquid CO2 to convert methane hydrates and release CH4 gases while absorbing CO2. This may represent a cost-effective and environmentally safe method for simultaneous CO2 sequestration and CH4 production during conversion of the hydrates.
The conversion of CH4 hydrate into CO2 hydrate is a near thermo-neutral process, and, therefore, there is no need to supply heat to the hydrates. Although CO2 hydrate is known to be slightly more stable than CH4 hydrate under the same pressures and temperatures, this does not mean that the conversion is straightforward. For example, too much pumped liquid CO2 may fracture the rock, whereas too little liquid CO2 may lead to conversion only on the exterior of CH4 crystals. One way to overcome this problem is by adding acid catalysts to the pumped liquid CO2 to speed up the conversion from liquid CO2 and methane hydrate to CO2 hydrate and gaseous methane. For example, U.S. Pat. No. 6,733,573, issued to Lyon, discloses the use of acid catalysts to accelerate the conversion. However, this method also comes with its own challenges, e.g., too much of acid catalysts may cause the wellbore to collapse, etc.
Therefore, there is a need to monitor both the downhole conversion of CH4 hydrate into CO2 hydrate and the ratios of the two components in fluids produced to the surface. To ensure efficient operations, it would be appropriate to also monitor the production of water from any of the zones, because water production may cause the rapid formation of hydrates, which may plug the wellbore.